The present invention relates to a process for manufacturing lithium secondary batteries, especially positive active materials to be used for the batteries.
Along with the rapid development of portable or cordless electric devices and appliances, there is an increased demand for compact and light weight secondary batteries having high energy density. In this respect, the lithium secondary batteries create greater expectations because of their high voltage and high energy density. Recently, lithium secondary batteries, so-called "lithium-ion batteries", having been developed employing LiCoO.sub.2 as the positive electrodes and carbon materials as the negative electrodes. The high voltages of these batteries are due to the operating positive electrode potential of LiCoO.sub.2 which is as high as 4 V compared to Li electrode potential. Batteries using carbon materials as negative electrodes have better charge-discharge cycle life characteristics and safety than the batteries using metallic Li negative electrodes, because the carbon material used for negative electrodes utilize the intercalation reaction which does not cause dendritic and/or mossy deposition of Li.
In view of the supply and cost of Co, efforts have been made to develop lithium containing oxides to replace LiCoO.sub.2. LiNiO.sub.2 has been identified as a potential replacement. LiNiO.sub.2, LiCoO.sub.2 and like kinds of lithium containing oxides provide high cell voltages and belong to the same class of compounds having hexagonal crystal structure for which the intercalation reaction is available. These are the expected candidates for positive active material. From this point of view, here have been proposals to use materials such as Li.sub.x NiO.sub.2 (U.S. Pat. No. 4,302,518), Li.sub.y N.sub.(2-y) O.sub.2 (Japanese Laid Open Patent Application No. Hei2-40861). Both are related to LiNiO.sub.2 ; and Li.sub.y Ni.sub.x Co.sub.(1-x) O.sub.2 as claimed in Japanese Laid Open Patent Application No. Sho63-299056; Li.sub.y Ni.sub.(1-x) M.sub.x O.sub.2 (where, M is one of Ti, V, Mn or Fe), which are lithium containing oxides wherein a part of Ni the of LiNiO.sub.2 is replaced with another metal. Other lithium containing oxides are also proposed, which include A.sub.x M.sub.y N.sub.z O.sub.2 (where, A is an alkaline metal, M is a transition metal, N is selected from the group of Al, In or Sn) (Japanese Laid Open Patent Application No. Sho62-90863); Li.sub.x M.sub.y N.sub.z O.sub.2 (where, M is at least one selected from the group of Fe, Co, Ni; N at least one selected from the group of Ti, V, Cr, Mn) Japanese Laid Open Patent Application No. Hei4-267053). Utilizing these materials, the active materials have been made to develop high energy density lithium secondary batteries of the class having 4 V discharge voltage.
Among these lithium containing oxides, LiNiO.sub.2 also has a 4 V operating positive electrode potential compared to a lithium electrode, and results in a secondary battery of high energy density. However, the capacities of the batteries using LiNiO.sub.2 positive electrodes deteriorate as the charge-discharge cycles proceed, down to 65% of the initial capacity after the 50th cycle. The unsatisfactory charge-discharge cycle characteristic is a problem.
As described above, in order to solve the problem, lithium containing oxides in which a part of Ni is replaced with another metal have been proposed. The materials in which a part of the Ni in LiNiO.sub.2 was replaced with another metal show relatively improved charge-discharge cycle reversibility, but the discharge capacity and voltage tend to go down. Thus, the principally desired high voltage, high energy density, characteristics are diminished. Among these proposals, the one in which a part of Ni is replaced with Co or Mn shows relatively favorable results in each characteristic for charge-discharge cycle reversibility, discharge capacity and voltage, as compared with conventional lithium containing oxides.
Active materials in which a part of Ni in LiNiO.sub.2 was replaced with Co are generally prepared by adding Co compounds like cobalt hydroxide, Li compounds like lithium hydroxide and Ni compounds like nickel hydroxide, and then burning them (herein after referred to as the mixing and burning period). The burning temperature for obtaining compounds having monophase crystal structure where the Co is replaced is to be within a range of 600.degree. C. to 800.degree. C. Growth of the monophase depends on burning temperature as the reaction is not complete at temperatures under 600.degree. C. As the Ni content increases, the single phase is created but the crystalline property deteriorates at temperatures exceeding 800.degree. C. It seems that at a temperature higher than 800.degree. C., Ni and Co atoms locate in the sites in the crystal where Li atoms should be, causing a disordered structure.
Active materials in which a part of the Ni in LiNiO.sub.2 was replaced with Mn are generally prepared by adding Mn compounds like manganese dioxide, manganese nitrate, Li compounds like lithium hydroxide and Ni compounds like nickel hydroxide, and then burning them using the mixing and burning method. In this mixing and burning method, however, a burning temperature higher than 800.degree. C. is needed to replace a part of the Ni completely with Mn. At a temperature lower than 800.degree. C. the substitution reaction is not completed insofar as can be estimated using X-ray diffraction techniques.
However, when it is burned at a temperature higher than 800.degree. C., Ni an Mn atoms locate in the sites in the crystal where Li atoms should be, causing disorder, deterioration of the ability to have the desired number of charge-discharge cycles and discharge capacity. Therefore, burning a lithium containing oxide based on LiNiO.sub.2 at a high temperature is not recommended.
Even if burned at any temperature range the discharge capacity tends to follow a decreasing trend as the charge-discharge cycles proceed, when the conventional manufacturing process (mixing and burning method) is used.